Pressurized-water nuclear reactors comprise a core consisting of prism-shaped assemblies arranged side by side in vertical position. The assemblies comprise a framework which is closed by means of end nozzles and in which are arranged the fuel rods held by spacer grids spaced apart from one another in the longitudinal direction of the assembly.
The spacer grids constitute a regular network, some locations of which are occupied by guide tubes intended for receiving the absorbent rods of control clusters ensuring the control of the power released by the core of the nuclear reactor. At least some of the guide tubes are attached to the two end nozzles of the assembly by means of their end parts and ensure the junction between the components of the framework and the rigidity of this framework.
One of the end nozzles of the assemblies, called the lower end nozzle, comes to rest on the lower core plate which is pierced with holes in the region of each of the assemblies, to allow the coolant water of the reactor to pass through the core in the vertical direction from the bottom upwards.
The coolant flow for the fuel rods passes through the adaptor plate of the lower end nozzle via apertures called water passages, which are either circular (of a diameter approximately 7 to 10 mm) or of oblong (apertures approximately 10 mm wide by 15 to 50 mm long). Debris which may be present in the primary circuit of the reactor is liable to be carried along by the circulating pressurized water, and if it is of small size (for example, less than 10 mm), this debris can pass through the adaptor plate of the lower end nozzle, the water passages of which have a large cross-section. This debris can become jammed between the fuel rods and the cells of the first grid, i.e., of the lowermost spacer grid holding the rods in a regular array. This debris, subjected to the axial and transverse hydraulic stresses which are high in this zone, can produce wear on the jacket of the fuel rod resulting in a loss of sealing of this jacket and an increase in the rate of activity of the primary circuit of the reactor.
Devices for filtering the coolant flow of the reactor, either during hot-running tests or during the operation of the reactor, are known in the art.
In the case of hot-running tests, the filter elements can be attached to the lower core plate and arranged thereon in the position of the fuel assemblies, before fuelling of the core, as described, for example, in FR-A-2,577,345.
In filtering during reactor operation, the filter elements are associated with the fuel assemblies and are generally arranged in their lower end nozzle. The filter elements fastened in the lower end nozzles of the fuel assemblies usually consist of sheet-metal or metal-wire structures making it possible to stop debris of a size smaller than the largest dimension of the cross-section of the passage between a fuel rod and a grid cell.
Such filter elements are described, for example, in U.S. Pat. No. 4,664,880, U.S. Pat. No. 4,684,496 and EP-A-0,196,611.
Such devices can be complex and introduce a relatively high head loss into the circulation of the coolant flow through the fuel assembly.
Furthermore, these devices placed in the lower end nozzle of the assembly can be bulky and obstructive during loading and unloading of the core assemblies and during the dismounting and refitting of the guide tubes and lower end nozzle.
Some fuel assemblies of recent design comprise a set of guide tubes (for example 16 out of 24) which are attached both to the upper end nozzle and to the lower end nozzle of the assembly, the rigidity of which they ensure, and a set of guide tubes which are attached to the upper connector only, of the assembly. The guide tubes of the second set which are freely engaged in the adaptor plate of the lower end nozzle perform only a guide function, and the lower end nozzle has, in the extension of these tubes, apertures which remain free.